Extrusion is often an important finishing step in the manufacture of polyethylene polymers. At some point downstream of a polymerization reactor, but upstream of an extruder, a polyethylene polymer will typically exist in granular form. The extruder is used to incorporate additives into the polymer and to compound and pelletize the polymer prior to use in commercial applications.
An extrusion process can be used to tailor the melt elasticity of conventional unimodal resins as described in U.S. Pat. Nos. 6,454,976; 7,892,466 and PCT Appl. No. 2007/106417 or alternatively to homogenize high and low molecular weight fractions in bimodal or multimodal polyethylene resins as described in U.S. Pat. Nos. 7,714,072; 8,079,747; U.S. Pat. Appl. Pub. No. 2005/0127559, and PCT Appl. Nos 2013/062810 and 2011/101438.
An extruder can reduce gels in a polymeric material by optimizing extruder process design or by incorporating a filter which screens for gels of a specific size as discussed in U.S. Pat. Nos. 5,730,885; 6,485,662; 7,393,916 and Eur. Pat. No. 942951.
Among the most common extruder designs used in the production of polyethylene polymers are those employing a co-rotating twin screw extruder (a “co-TSE”) or a continuous internal mixer with counter-rotating rotors.
The general state of the art in co-rotating twin screw extrusion methods has been well catalogued in recent publications such as James L. White and Eung K. Kim in Twin Screw Extrusion: Technology and Principles (2nd Ed.) Carl Hanser Verlag, Munich 2010; Klemens Kohlgruber and Werner Wiedmann, in Co-rotating Twin-Screw Extruders: Fundamentals, Technology, and Applications, Hanser, Munich 2008; Chan I. Chung in Extrusion of Polymers: Theory and Practice, Carl Hanser Verlag, Munich 2000; and Paul Anderson in Mixing and Compounding of Polymers (2nd Ed), Ed. Manas-Zloczower, Tadmore 2009, Chapter 25, p. 947. Nevertheless, extruder set-up and operation is open to manipulation in order to operate best with a certain polymer type, optimize polymer properties, and to optimize polymer throughput rates. Hence an extruder process can be designed to: maximize polyethylene polymer throughput, minimize polyethylene polymer gel content, and/or improve polyethylene resin performance attributes such as resistance to oxidation, slow crack growth resistance and impact toughness.
Gels in polyethylene polymers generally correspond to localized polymer domains where optical and/or physical properties are significantly different from the polymer bulk, and are generally considered to be defects. When a polyethylene resin is made into for example a film, the gels will typically have different optical properties, allowing them to be identified and quantified with instrumentation such as an OCS gel camera. Gel content is often an important quality control parameter for the production of polyethylene resins.
As discussed above, gels can be eliminated or reduced by using a filter screen in combination with an extruder. Although this method works well for gels which consist of cross-linked polymer or foreign substances, it does not work well with high density polyethylene polymers of high molecular weight or multimodal polyethylene resins, in which polymer inhomogeneity leads to gel formation. Polymers containing high molecular weight fractions produce a high viscosity polymer melt during extrusion. High viscosities lead to a large pressure drop across the filter screen and can drop the polymer throughput to unacceptable levels. A high polymer melt viscosity can also increase the polymer melt temperature to a level beyond which polymer degradation occurs and the polymer properties are compromised.
To address this problem, gels have been reduced by using a modified extruder design. For example, use of a continuous mixer that includes an additional “independently controlled” secondary mixer to enhance mixing, substantially removes gels as discussed in U.S. Pat. No. 5,458,474.
Alternatively, and as shown in U.S. Pat. Appl. Pub. No. 2006/0245294, an extruder consisting of two co-rotating twin screw extruders where the downstream extruder is dedicated to mild kneading can be used to reduce gels. The mild kneading section which is a dispersive mixing zone reduces gels that are mainly un-melted nascent polymer particles or undispersed polymer components of high molecular weight. This extruder design, known commercially as ZSK-NT™ effectively separates the extrusion into two stages: in the first stage, the polymer is gently melted; in the second stage, the high molecular weight polymer fractions are homogenized through a dispersive mixing process.
Despite these successes, the forgoing extruders are far more expensive than conventional twin screw extruders. Hence, it would be advantageous to provide a polymer extrusion method which achieves good polymer properties and high extruder throughputs by manipulating a conventional co-rotating twin screw extruder.
It is known that to induce dispersive mixing, extensional or elongational type polymer flows are necessary. In a co-rotating twin screw extruder this may be effectively achieved by subjecting the polymer melt to specific kneading elements. A dispersive mixing kneading element can be a combination of multiple kneading blocks which are typically staggered disks forming a fixed angle to one another. Such a kneading element may also comprise a single unit that has a geometry equivalent to that of a number of staggered disks. These kneading disks normally have a contoured surface that is not parallel to the screw axis and which bears one or more edges or vortices that present minimal clearance relative to extruder barrel together with surfaces positioned at a greater distance from the barrel. The disk and the barrel thus form a flow channel that forces the polymer melt into alternatingly expanding and contracting when the screws are rotating. The extensional flow thus generated is efficient for dispersive type mixing which can reduce gels. While the extensional flows generated by these kneading disks are the most efficient manner by which to generate dispersive mixing, the use of such elements is limited in the following ways. First, these kneading elements generally lead to high energy input and hence high melt temperature, especially in extruders used for commercial polyolefin production where heat is not effectively removed due to the size of the extruder. The excessive heat may in turn result in polymer degradation. To avoid overheating the resin, the screws have to be operated at reduced speed. The reduced screw speed, however, leads to lower polymer throughput because melting capability is proportional to the screw speed for given screw configuration.
One manner by which to alleviate the temperature rise is to use kneading elements having a different profile. For example, as disclosed in Eur. Pat. Appl. No. 2,374,600, kneading elements designed for reduced shear at the extruder barrel can be used. This approach, however, may have its own limitations in that the design would employ a larger clearance between the kneading elements and the extruder barrel which can reduce the overall pumping efficiency of the extruder.
A different limitation is encountered if extra dispersive mixing kneading disks are employed. As the kneading disks are usually fully filled, they cause higher torque and specific mechanical energy at the same level of throughput. Since the power or torque allowed for the given extruder is limited, the polymer throughput rate can be similarly limited.
New extruder designs are necessary to reduce defects, while minimizing cost and maintaining high throughputs. This is especially true for polyethylene in which distinct high and low molecular weight fractions are present as they can be difficult to homogenize, leading to gels or defects in the finished resin. Such resins often find applications in high pressure pipe, blow molding formulations and a wide range of blown films.